Ruud error code 10 means the R921T furnace is in a one-hour lockout. Look for the second code displayed with it because that code identifies the underlying problem.
Intermediate to advanced
Error code 10 means the furnace control has entered a one-hour lockout after a serious or repeatedly occurring fault. The display should alternate code 10 with the underlying code that caused the lockout.
No gas heat during the lockout period; cooling or fan modes may still operate; code 10 alternates with another fault code; the furnace may retry heating after the lockout expires.
Four consecutive failed ignition attempts; repeated water-sensed faults; blower unable to run after an open-limit condition; unexpected flame; internal control fault.
1. Record every code shown, including the code alternating with 10. 2. Do not repeatedly reset power because the companion code identifies the root problem. 3. Check only safe external items such as the thermostat setting, air filter, open registers, visible vent terminations, and whether the manual gas shutoff valve is open. 4. Review the Furnace Lockout Troubleshooting Guide at /furnace-lockout-troubleshooting/. 5. A qualified technician should diagnose the companion code before the furnace is returned to service.
Integrated furnace control; component associated with the companion fault; ignition system; condensate system; blower system; gas valve; venting and safety controls.
Yes if code 10 returns, another critical code is displayed with it, or the furnace does not resume normal operation after the lockout.
Do not repeatedly reset a locked-out furnace. Turn the system off and seek service immediately if you smell gas, see soot, or suspect overheating or flame rollout.
Learn more about the furnace components and operating conditions commonly associated with diagnostic error codes: